It has long been known to nitrate aromatic and substituted aromatic compositions for producing a variety of products which are useful as intermediates in the chemical industry. For example, toluene is nitrated to form an intermediate, i.e., paranitrotoluene, which is useful for the preparation of dyestuffs and drugs intermediates. A dinitrotoluene, a mixture of 2, 4 and 2,6-dinitrotoluene has been employed in the manufacture of toluene diisocyanate which is useful for the formation of polyurethanes.
It is known that in the nitration of mono-substituted aromatic compositions that 3 isomers can be formed and the proportion of each isomer formed often is largely dependent upon the functional group present on the aromatic ring. For example, when toluene is nitrated, approximately 58% of the ortho-isomer, 38% of the para-isomer and 4% of the metaisomer are formed whereas when chlorobenzene is nitrated about 30% of the product is the ortho-isomer and 70% is the para-isomer. Even though specific groups on the aromatic ring can assist in the formation of a larger proportion of appropriate isomers, these groups may not be desired in the final product or they may not provide sufficient selectivity.
The presence of a plurality of isomers in the nitration mixture may be undesirable and can lead to economic waste in terms of the materials consumed and in terms of recovery or disposal. For example, with respect to nitrotoluene, paranitrotoluene is useful, while the ortho and meta-nitrotoluene isomers are not as useful, in the preparation of dyestuffs and drug intermediates. Thus, these isomers must be removed from the product which adds to the cost.
For end uses in the polyurethane industry a high ratio of 2, 4-dinitrotoluene to 2, 6-dinitrotoluene is desired. Some 2, 6-isomer of dinitrotoluene is produced by nitrating orthonitrotoluene whereas solely the 2, 4-dinitrotoluene isomer is produced from para-nitrotoluene which again shows the importance of obtaining para-nitrotoluene. The isomers produced from meta-nitrotoluene are unacceptable in many of the polyurethane applications and thereby represent a problem in terms of purification, i.e., removal of the meta-nitrotoluene from the product.
Another substituted aromatic compound, i.e., 4-nitro ortho-xylene is highly useful and desirable in many applications in the chemical industry. When orthoxylene is conventionally nitrated, approximately equal amounts of 4-nitro ortho-xylene and 3-nitro ortho-xylene are formed. The 3-nitro isomer generally is removed from the product which adds to the cost and leads to waste.